Cardiotrophin-like cytokine (CLC) is a novel member of the interleukin (IL)-6 cytokine family. Currently, the IL-6 cytokine family consists of IL-6, IL-11, cardiotrophin (CT)-1, ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), and oncostatin-M (OSM). Each of these factors can induce a wide variety of growth and differentiation activities through their interaction with receptors consisting, in part, of a common signaling subunit designated gp 130 (Kishimoto, T., et al., Blood 86:1243-1254; 1995). For high affinity receptor binding, IL-6 and IL-11 induce the homodimerization of gp 130. IL-6-binding then recruits a protein termed the IL-6 binding chain, or gp80, to the IL-6/receptor complex (Murakami, M., et al., Science 260:1808-1810; 1993). On the other hand, CT-1, LIF, and OSM induce gp130 heterodimerization with the 190 kDa low affinity LIF receptor b (also designated gp190; Davis, S., et al., Science 260:1805-1808; 1993; Gearing, D. P., et al., EMBO J. 10:2839-2848; 1991). CNTF-binding also involves the interaction of a third cellular protein, designated the a-CNTF receptor subunit, with the gp 130/gp 190 heterodimer (Davis, S., et al., Science 253:59-63; 1991). Additional receptor binding subunits have also been identified in the cases of IL-11(Hilton, D. J., et al., EMBO J. 13:4765-4775; 1994) and OSM (Thoma, B., et al., J. Biol. Chem. 269:6215-6222; 1994), thus promoting an interest in the continued characterization of the interactions of IL-6 cytokine family members with their respective receptor complexes. In any case, induction of gp 130 receptor complex formation by IL-6 cytokine family member binding is followed by the activation of one or more cellular signal transduction pathways especially including the Jak/STAT pathway (Darnell, J. E., et al., Science 264:1415-1421; 1994).
The members of the IL-6 family exhibit many shared biological activities including the activation of hepatocellular transcription, the activation of neural cell proliferation and differentiation, and the regulation of hematopoiesis (Baumann, H., et al., J. Biol. Chem. 268:8414-8417; 1993; Yamamori, T., et al., Science 246:1412-1416; 1989; Leary, A. G., et al., Blood 75:1960-1964; 1990). Furthermore, CT-1, LIF, CNTF, and OSM have been detected in the early stages of development and also appear to allow embryonic stem cells to grow in an undifferentiated state in vitro (Smith, A. G., et al., Nature 336:688-690; 1988; Conover, J. C., et al., Development 119:559-565; 1993; Pennica, D., et al., J. Biol. Chem. 270:10915-10922; 1995).
A number of specific functions have also been ascribed to various members of the IL-6 cytokine family in addition to the common activities listed above. For example, OSM inhibits the growth of A375 human histiocytic melanoma cells and several types of lung, breast, ovary, and stomach tumor cell lines (Zarling, J. M., et al., Proc. Natl. Acad. Sci. USA 83:9739-9743; 1986; Brown, T. J., et al., J. Immunol. 139:2977-2983; 1987; Malik, N., et al., Mol. Cell. Biol. 9:2847-2853; 1989; Horn, D., et al., Growth Factors 2:157-165; 1990). Furthermore, OSM stimulates the growth of normal fibroblast cell lines, rabbit vascular smooth cells, bovine aortic endothelial cells, and Kaposi's sarcoma-derived spindle cells (Grove, R. I., et al., Proc. Natl., Acad. Sci. USA 90:823-827; 1993; Nair, B. C., et al., Science 255:1430-1432; 1992; Radka, S. F., et al., J. Immunol. 150:5195-5201; 1993).
Cardiotrophin-1, or CT-1, is the member of the IL-6 cytokine family most closely related by sequence identity to the amino acid sequence of the gene of the present invention. CT-1 was identified by combining an expression cloning approach with an embryonic stem cell-based model of in vitro cardiogenesis (Sheng, Z., et al., Development 122:419-428; 1996; Pennica, D., et al., Proc. Natl. Acad. Sci. USA 92:1142-1146; 1995; Ishikawa, M., et al., Biochim. Biophys. Res. Comm. 219:377-381; 1996). Adult cardiac muscle is terminally differentiated and, unlike skeletal muscle, cardiac muscle tissue does not contain muscle cells which retain their proliferative capacity. As a result, injury to the heart muscle is often irreversible and results in scarring and ultimately in an overall decrease in heart function.
In response to mechanical stimuli and hemodynamic stress, the adult myocardium can activate an "adaptive hypertrophic response" characterized by an increase in myocardial cell size, but not in cell number (Chien, K. R., et al., Annu. Rev. Physiol. 55:77-95; 1993). The specific type of hypertrophic response can be modulated by mechanical induction of an excess of pressure or an excess of volume on the cardiac system. During extended periods of hypertension (that is, excessive pressure on the system), a distinct form of myocardial cell hypertrophy can result in which individual cardiac myocytes increase in length, but not in diameter (Anversa, P., et al., Circ. Res. 52:57-64; 1983; Gerdes, A. M., et al., Lab. Invest. 59:857-861; 1988). The overall effect of excessive volume hypertrophy can be scarring, fibrosis, and the loss of viable cardiac myocytes throughout the heart, and, ultimately, an irreversible loss of heart function (Wollert, K. C., et al., J. Biol. Chem. 271:9535-9545; 1996), while the overall effect of pressure hypertrophy is usually characterized by a preservation of contractile function and is often reversible.
CT-1 was isolated in an attempt to identify factors which are involved in the above-described process of excessive volume-mediated myocardial cell hypertrophy. In addition, it has been observed that CT-1 can promote the survival of ventricular muscle cells in vitro (Sheng, Z., et al., Development 122:419-428; 1996). Currently, it is hypothesized that CT-1 may affect a survival phenotype on such muscle cells in culture in vitro through a mechanism in which CT-1 activates cellular signal transduction pathways which can block apoptosis of cardiac myocytes.
Thus, there is a need for polypeptides that function as mediators of cardiac hypertrophy, since disturbances of such regulation may be involved in disorders relating to the heart including arrhythmia, inotropia, heart failure, cardiomyophathy, hypertrophy, ischemia, hypertension, valvular or pericardial abnormalities. Therefore, there is a need for identification and characterization of such human polypeptides which can play a role in detecting, preventing, ameliorating or correcting such disorders.